1. Field of the Invention
The present invention relates to a plating method for forming a composite structure including a plating film on a metal base substrate, and in particular to composite structures suitable for use as or as components in car bumpers, rear-view mirrors, reflectors, electric and electronic parts, precision machine parts, air plane parts, engine pistons, bus-bars, electrical cables, and the like.
2. Description of the Related Art
The preparation of various products by plating substrates prepared from iron and aluminum base materials with a plating film is well known in the art. According to conventional processes, the plating step is typically preceded by pretreatment of the metallic base material which forms the substrate. The pretreatment usually involves removing oxide film and stains from the surface of the material. Oxide films adversely deteriorate the adhesive strength between the metallic base material and the plating film; therefore, removal of the oxide films by pretreatment improves the adhesive strength between the plating film and the metallic base material.
Aluminum base material is more likely to be oxidized by air and develop an oxide film thereon than most other metal base materials. However, even if an oxide film is removed from the surface of aluminum material, the surface will re-oxidize during the plating process and adversely affect the adhesive strength between the metallic base material substrate and the plating film.
To avoid the above drawback, especially in relation to substrates formed from aluminum base materials, a zinc immersion process has been used for preventing re-oxidization of aluminum base materials. The zinc immersion process is usually performed as follows. First, the surface of an aluminum base material is degreased by solvent degreasing and alkaline degreasing. Then, the surface of the material is etched with an etchant. The principal component of the etchant is sodium hydroxide. Since the etched base material contains various kinds of impurities, copper and magnesium smuts or stains are prone to be formed on the surface. The smuts need to be removed for the plating film to adhere well to the base material. A smut removing treatment is thus performed on the base material with an acid such as an acid selected from the group consisting of nitric acid, hydrofluoric acid and sulfuric acid.
Once the smuts are removed from the aluminum base material by acid treatment, the treated aluminum base material is subjected to a zinc immersion process (or a zinc alloy immersion process). In this immersion process, the base material is processed in a zinc immersion solution. The principal component of the solution is sodium hydroxide and zinc oxide. This process removes the thin oxide film on the surface of the base material and forms a zinc film on the exposed surface. The formed zinc film is removed with nitric acid. The base material is then subjected to another immersion process, which forms a zinc film having a more uniform thickness.
After performing each of the above-mentioned pretreatment steps, the aluminum base material covered with the zinc film is subjected to a known electroplating process. In the electroplating process, the base material is immersed in a plating solution and a voltage is applied between electrodes. This forms an electroplating film on the surface of the base material.
As is apparent from the foregoing description, the above-described conventional plating method requires many steps (often more than ten steps, including the pretreatment and the electroplating) for forming a plating film having a sufficient peel strength (or delamination strength) on the surface of the base material substrate. The large number of steps associated with this conventional plating method complicates the plating procedure and commands the provision of an extremely large facility in order to accommodate all of the equipment needed to perform each of these steps.
The disadvantages associated with the above-discussed conventional method are compounded even further where a plating film having a plurality of regions each containing a different composition (e.g., a different concentration of insoluble particles or metallic components or mixtures of plating solution) is prepared. For example, it is sometimes desirable to prepare a plating layer having a first region proximal to the outer surface of the film with a certain composition, and a second region proximal to the opposing surface contacting the base material with a different composition. In this case, a plurality of separate plating solutions, with each plating solution having a different composition from the others, is needed to prepare the different regions of plating layer. Similarly, where several types of products are prepared, and each of the product types contains a plating film having a specific metal composition that differs from the metal compositions of the other product types, the provision of a different plating solution for each product type is required. The provision of a plurality of plating baths to obtain the above-discussed variations of plating films in accordance with conventional techniques further complicates the plating procedure and still further increases the cost for and space needed in the production facility.
In the above-discussed conventional electroplating methods, after pretreatment is completed the aluminum base material is immersed in a plating solution, and a voltage is applied between electrodes. This forms an electroplating film 52 on the surface of a base material 51 as shown in FIG. 14.
As further illustrated by the arrows in FIG. 14, the plating film 52 electroplated on the base material 51 develops an internal residual stress in the shrinking or compressive direction. The residual stress is believed to be attributable to the absorption of hydrogen atoms by metal ions in the plating solution when the plating film 52 is being formed. The absorbed hydrogen atoms generate hydrogen gas, which is expelled as the plating film 52 is being deposited. The expulsion of the hydrogen gas provides the plating film 52 with a microscopically porous structure, thereby generating a compressive force toward the center portion of the plating film 52. The residual stress in the shrinking direction is thus generated. The residual stress in the plating film 52 can lead to the formation of cracks in the plating film 52 or cause the film 52 to delaminate from the base material 51.
If the material 51 has a narrow groove 53 as shown in FIG. 15 or a recess, it is difficult to introduce the plating solution into the groove 53, and especially difficult to contact the solution to the bottom of the groove 53, by practicing the conventional immersion technique. Consequently, the groove 53 often is not covered with the electroplating film 52 when coated in accordance with conventional processes. Accordingly, the prior art method often fails to produce the desired plated products.
A need therefore exists to provide a process for making a composite structure comprising a substrate formed from a metal base material and a plating film electroplated on the metal base material, in which the composite structure can be produced in a more efficient and cost effective manner, and in which the resulting composite structure is not prone to cracking or delamination and has the plating film formed in narrow grooves and recesses.